1. Field of the Invention
The present invention relates to an ion implantation equipment.
2. Description of the Related Art
In the ion implantation equipment, as a method for implanting an ion, there is used a method in which impurity material is ionized and the ion is implanted into a semiconductor substrate, for example, a semiconductor wafer with high energy. In this case, in the ion implantation equipment, there are a medium current type, in which beam current of ion is 2 mA or less, and a high current type, in which beam current of ion is 2 mA or more. For example, the high current type is constructed as shown in FIG. 4.
Specifically, FIG. 4 shows the outline of the entire ion implantation equipment. A part of necessary energy is supplied to an ion 1, which is drawn from an ion source 2, from an accelerating tube 3. Thereafter, the ion is introduced into a mass spectrograph 4 and a required ion is selected. The selected ion 1 is accelerated by an accelerating tube 5 and implanted into the semiconductor wafer mounted on a disc 10, which is a rotational mount base.
In a case where ion-implantation is performed in a semiconductor wafer 20 as mentioned above, oxide layer, which is exposed on the surface of the wafer 20, is charged up to be generated a positive charge of ion 1. If an amount of surface charge, which is generated by the above-mentioned charging, is more than an amount of the insulation breakage charge, the insulating film 20 is broken. The insulation breakage and failure occur remarkably as degree of integration becomes higher. This is a big subject matter to be overcome in IC which is developed from IC of 1M to 4M, 16M and to 64M.
Due to this, in conventional, there is generally used a method, in which the irradiation of electron, which is called an electron flood, is performed in the wafer 20 simultaneously with the implantation of ion 1 in Faraday 6, and the positive charge is neutralized, thereby preventing from being charged up (U.S. Pat. No. 4249077, Published Unexamined Japanese Patent Application Nos. 58-166930 and 64-86435).
In other words, a thermoelectron (primary electron e.sub.1), which is generated by a filament, is drawn from the filament at the speed in accordance with an accelerating voltage (for example, 300 V), which is a potential difference between the filament, which is the first electrode discharging electrode, and a wall surface of an opposite metal electrode, and collides with the wall surface of the opposite metal electrode. Thereby, a secondary electron e.sub.2 is emitted from the wall surface of the opposite metal electrode. The discharged secondary electron e.sub.2 is incident on the semiconductor wafer so as to neutralize the surface charge, which is generated in the insulating film region.
In this case, an amount of the generation of the secondary electron responds to the potential difference between the primary electron discharge electrode and the opposite metal electrode for the secondary electro discharge, and is fixed in accordance with an amount of the ion beam current.
In the electron shower, the following demerit is found:
As shown in FIG. 5, since the energy distribution of the secondary electron includes high energy components such as Auger electron and a diffraction electron, the electron with high energy collides with the insulating film of the wafer and a negative chargeup is generated.
Also, when the ion implantation is performed, the insulating film of the semiconductor wafer is sputtered by ion beam and becomes splash. The splash is adhered to the inner wall surface of Faraday, which is the ion implantation chamber, thereby forming an insulating film on the inner wall surface. Due to this insulating film, a negative charge is charged up on the inner wall surface, and the potential of the opposite metal electrode of the filament changes to the negative side.
As mentioned above, when the potential of the opposite metal electrode changes, the energy of the electron reaching to the wafer is increased than the initial energy. Then, the electron with high energy collides with the insulating film region of the surface of the semiconductor wafer, on which ion beam is not irradiated, thereby generating the negative charge up.
In other words, in the electron flood, the accelerating voltage, for example, high voltage of 300 V is applied to the primary electron. However, since the accelerated primary electron was often directly incident on the surface of the semiconductor wafer, the surface of the semiconductor wafer was often damaged.